damp in concrete plinth

[bitfr]

In the passageway between me and neighbour, I have a shaded flank wall with render plinth that shows evidence of rising damp.
//www.diynot.com/network/bitfr/albums/24408/91933

As can be seen in the photo, the damp raises even when there is no standing water in passage. The passage itself is concreted out, (my) wall to (neighbour) wall. On the inside of the house I have damp rising from the floor, some on the wall itself, but most on the chimney breast (which obviously is some distance from the plinth...).

I had a RICS survey done and it was recommended I remove the porous plinth and reduce the ground level in the passage by cutting a gully in the passage next to my wall, and fill it with shingles.

My questions: are there any disadvantages to removing the plinth? Will the gully not soak up all the rain water and hence, increase the damp of the wall? I'd like to avoid lifting all of the concrete in the passage...

 

[ree]

1. There are no disadvantages to removing the plinth. You would be best advised to remove all the S&C plinth, clean the brick face and repoint. Then wait and see if a French drain is needed.

2. how is the passageway drained of rainwater - is the concrete surface sloping or running any particular way?

3. Is the interior floor suspended or solid?

4. is the wall solid or cavity? I suspect solid.

5. Did the RICS check for damp around all your walls?

6. Why not post pics of the full passage and pics of the interior damp.

 

[Joe Malone]

There is no benefit to having the plinth in the first place. All it does is prevent wall base ventilation and therefore evaporation of the moisture from the wall base. Prior to DPC's coming into common use is was not unusual to see brick plinths at wall base finished with a course of plinth stretchers. This did significantly help reduce the incidence of wall base damp and is a very effective feature. However, at some point someone thought they would run with a good idea and ruin it by stretching the idea to include retrofit cementitious plinths. These are not a good idea, they inhibit wall base evaporation and wick up moisture from the ground by diffusion. It is also not unusual to see DPC's bridged by these plinths. Take it off and do nothing for 6 months, the wall may well dry out in the interim.

 

[bitfr]

Many thanks for the replies.

Can I ask for the reason for such cement plinth in the first place? Seems almost every house has them in the London area...

To answer the questions posed by ree:
1. Would you suggest a French drain in preference to gully filled with shingles? I understand a French drain is sealed from the bottom, whilst the gully wouldn't be.
2. The passage is bit below the street level and same level as backyard. When it rains, I see bit of standing water (not excessive), especially close to the street
3. The house floor is suspended
4. The wall is solid, no cavity
5. RICS surveyor investigated mainly the areas I indicated, ie. flank and front wall.

Sounds like the plinth needs to go first, but I should also consider draining the passage way, especially close to the street. How would such drain direct the water away from the the house walls?

I've upladed more photos of the plinth and the passageway:
//www.diynot.com/network/bitfr/albums/24408

 

[Joe Malone]

Can I refer you back to my first post where I did explain why cement plinths were installed. If someone takes the time to give you advice then at least have the courtesy to read it.

 

[bitfr]

Cheers Joe. Whilst I understand the idea of minimize the costs of a proper brick plinth, I wonder why everyone has adopted the cement if it's such a bad idea... In my area ~80% of the houses have the cement plinth, the rest have no plinth. If the original purpose of the plinth was to prevent damage from rain/stone splashback - would it make sense to replace the current cement with eg. waterproof cement?

Also, any opinion on the draining ditch filled with shingles?

I'm also adding a photo of the flooded passageway. The water isn't actually touching my side (left hand side), which makes me think the plinth takes on the moisture from the ground - again, would waterproof cement be of help?

//www.diynot.com/network/bitfr/albums/24408/92053

 

[Nige F]

Many thanks for the replies.

Can I ask for the reason for such cement plinth in the first place? Seems almost every house has them in the London area...

]

Reason is probably fashion in the trade @ a certain time - (another for example is the practice of running waste pipes into a channel and then into a gulley above a grid, building regs. way back. Then later the channel went .discharge of waste above a grid is fine IF it's cleaned often. Leave it for decades and add condensate from a new boiler and it PONGS- just sorted it for a neighbour who had a mystery smell ) I think that in time "breathable membranes" in roofs will be added to the list of bad ideas in buildings

 

[Joe Malone]

Cheers Joe. Whilst I understand the idea of minimize the costs of a proper brick plinth, I wonder why everyone has adopted the cement if it's such a bad idea... In my area ~80% of the houses have the cement plinth, the rest have no plinth. If the original purpose of the plinth was to prevent damage from rain/stone splashback - would it make sense to replace the current cement with eg. waterproof cement?

Also, any opinion on the draining ditch filled with shingles?

I'm also adding a photo of the flooded passageway. The water isn't actually touching my side (left hand side), which makes me think the plinth takes on the moisture from the ground - again, would waterproof cement be of help?

//www.diynot.com/network/bitfr/albums/24408/92053

I specialise in building defect analysis work, particularly damp investigation. My advice is simply to remove the cement plinth and be patient for 6 months to see if the problem improves. Cement plinths did become quite fashionable but it was always a bad idea and I've no doubt it was sold to homeowners in much the same way that chemical injection is sold these days; its all to do with marketing and very little to do with proper diagnosis and specification. Install a French drain and you install an ongoing maintenance obligation because the filter membrane needs changing every couple of years. This is simple stuff... just remember the three key requirements, ventilation, ventilation, ventilation, all at wall base though. The wall that best manages rising damp is a bare wall and any applied coatings inhibit moisture evaporation from the wall base which means it will rise further up the wall.

 

[bitfr]

Thanks Joe, you've convinced me, the plinth comes off today